A large number of analogs of adenosine or of adenosylhomocystein have been examined for their ability to function as inhibitors and/or substrates of adenosylhomocysteinase. The synthesis of analogs of adenosylhomocysteine by this enzyme in vivo may be used to form very potent and specific inhibitors of transmethylation reactions. These analogs have been shown to have a wide range of important biological activities, including antiviral activity against a number of RNA and DNA viruses, inhibition of viral transformation, inhibition of chemotaxis in mouse macrophage cell lines, and stimulation of cell differentiation in a number of cell lines. The ability of several adenosine analogs to inhibit and/or act as substrates for the adenosylhomocysteine hydrolase has been examined both in vitro, using purified enzyme from beef liver and hamster liver, and in vivo for a variety cell lines. The two most interesting compounds studied are 3-deazaadenosine and 3-deazaaristeromycin. Both are inhibitors of adenosylhomocysteinase, but only 3-deazaadenosine is a substrate for the enzyme. 3-Deazaadenosine, but not 3-deazaaristeromycin inhibits chemotaxis by a mouse macrophage cell line. In vivo, several methylation reactions, including phospholipid methylation, protein lysine and arginine methylation, and protein carboxyl methylation are inhibited to the same extent by both compounds. However, 3-deazaadenosine specifically inhibits synthesis of a small number of proteins, which may be due to inhibition of some reaction involved in mRNA synthesis or processing, such as methylation of the 5' cap. Differences in specificities of these compounds can be used to search for the function of specific methylation reactions in chemotaxis, RNA synthesis, and other cellular processes.